Field of the Invention
The present invention relates to a laser illumination device, and more particularly, to a partial random laser illumination system and device having a random phase and amplitude component for speckle suppression.
Description of the Prior Art
A laser projection system using a conventional high-coherence laser source generally causes laser speckle patterns on a rough screen surface to which the laser beam is projected. Laser speckle will degrade the image contrast and may irritate the eyes of the observer. To address this problem, one prior art approach to reducing laser speckle is to vibrate a ground-glass plate. Although this known technique can reduce laser speckle effectively, the light source forms a surface light source, which makes the laser source unable to be focused on one spot. In other words, the diffuser used in this technique affects the beam divergence angle. The divergence angle of the output laser beam cannot remain a small one (a divergence angle of about ±20 degree), and the resulted transmittance is only 50%, wasting considerable light energy. In view of the problems described, the above technique can be applied to projection displays based on LCOS (liquid crystal on silicon) or DLP (digital light processing) technologies only, but not to MEMS (micro-electro-mechanical system) devices that require small divergence angles and small light spots.
Another prior art approach employs the technique of vibrating periodic diffractive optical elements to reduce laser speckle. This technique can be applied to MEMS devices; however, the technique requires complicated design of optical elements and perfectly periodic structures, resulting in production difficulties as well as high costs. Yet another prior art approach employs random lasers to address the laser speckle problem. However, the low spatial coherence of a random laser makes it hard to focus the laser source on one spot perfectly.